1. Field of the Invention
The present invention relates to manufacture of electrical circuitry and more particularly concerns manufacture of electrical circuitry having multiple conductive circuit layers interposed within a dielectric substrate, which conductive layers are interconnected to one another through the substrate.
2. Description of Related Art
As electrical printed circuits are packaged in ever smaller volumes, density of circuit components increases, and it is common to form a single dielectric substrate with layers of circuitry on both of its sides and, further, to form stacks of such substrates and circuitry. Circuit layers on the two sides of a dielectric substrate are frequently connected to each other through a hole in the substrate to form a through hole connection. One common method of forming a through hole connection comprises initially plating the electrically non-conductive sides of the hole with an electroless copper or nickel copper coating, and thereafter electrolytically plating the hole so as to provide a continuous electrically conductive plating or coating through the hole from a pad on one side of the dielectric substrate to a pad on the other. Electroless plating is a laborious, time consuming process, frequently requiring from seven to twelve different tanks of solutions and rigid process controls.
Through hole technology requires drilling holes through the dielectric substrate to pads on both sides that are to be connected by the electroless and electrolytic plating. In manufacture of a stack of circuit boards, this method wastes valuable areas on the layers that are not necessarily involved in the connection, since the holes must be drilled through all layers of a stack of several circuit boards and not just through layers to be connected. An alternative method of interconnecting layers of a board in a stack of boards involves the use of buried vias which extend only through a single board of a stack of boards, for example, and connecting only specific layers desired. But this procedure requires sequential lamination of the several boards, reduces yield and increases costs by more than one order of magnitude.
Not only does the use of plating techniques require rigid controls on a great many chemical baths, but the process generates undesired and hazardous effluents. Thus, special handling of hazardous chemicals is required. Techniques for disposal of resulting hazardous effluents are complex and expensive and subject to strict government controls.
Accordingly, it is an object of the present invention to provide circuit layer interconnections that avoid or minimize above mentioned problems.